Mobile communications devices have become an integral part of society over the last two decades. Indeed, more than eighty-two percent of Americans own a mobile communications device. The typical mobile communications device includes an antenna, and a transceiver coupled to the antenna. The transceiver and the antenna cooperate to transmit and receive communications signals.
The typical transceiver includes a power amplifier for amplifying low voltage signals for transmission via the antenna, and a local oscillator providing a reference frequency signal. The typical mobile wireless communications device uses the reference frequency signal to demodulate a received signal transmitted at a carrier frequency for subsequent baseband processing. The reference frequency signal is also used in the uplink transmission path for modulating a baseband signal and preparing it for transmission.
A typical local oscillator may comprise a crystal oscillator. In these oscillators, the natural resonance of the crystal is used as a basis to create a known repeating electronic signal, such as a clock signal. For example, quartz crystals are commonly used in time instruments, such as a quartz movement timepiece. Typically, the resonance of the crystal is very accurate and provides a precise basis. Nevertheless, the performance of the crystal can be affected by aging and temperature conditions, which can adversely affect the performance of a communications device using the crystal for the basis of its local oscillator. For example, one common oscillator used in communications devices is a voltage controlled crystal oscillator.
Because of this variance in the oscillator, some communications devices apply a calibration approach. In other words, these communications device periodically calibrate their oscillators using another reference signal, hopefully immune to the temperature and aging effects. In one approach, the communications device is directed to transmit a test tone at a previously established frequency. The output of the communications device is recorded and tested for accuracy by external equipment. Any defects in the transmitted test tone are used to recalibrate the oscillator in the communications device. Because of the need to use external equipment, communications devices calibrated with this approach are usually taken out of service. In communications devices that include only receive capabilities, the above approach may not be a viable option.
Moreover, for devices used in the field, the typical approach of disassembly and using signal probes to test the local oscillator may not be practical. Indeed, in these approaches, the device is again temporarily put out of service and sent to an advanced repair facility.
In one approach disclosed in U.S. Patent Application No. 2012/0050100 to Huang et al., the communications device calibrates the oscillator using a global positioning system (GPS) device. In particular, the communications device uses a pulse per second signal from the GPS device to calibrate the oscillator.